A registry was established in 1972 and has been maintained to characterize and classify all spontaneous neoplasms from "veterinary using" dogs and cats housed within a defined geographic area. The defined area is Tulsa County, of 400,709 human population, and the surrounding smaller cities, to make over 500,000 population in a greater Tulsa area. The adjacent countryside is sparsely populated in human, dog and cat subjects with reasonable isolation of the dog and cat population. In collecting the tumors for statistical interpretation, the numerator is only of histologically diagnosed tumors. In the limits of the proposal, this criterion can only be met when tissue is submitted by a veterinarian. The appropriate denominator is the total "veterinary using" dog and cat population from hospital records of participating veterinarians. Licensing bureaus only provide a limited listing of dogs in Tulsa; hence hospital records are the only accurate records. Consistency in histogenetic classification of tumor is favored by our policy of restriction of tissue interpretation to three veterinary pathologists with interest in tumor histopathology, frequent consultation, and constant reference to previous cases and the use of cytologic criteria on tumor imprints and exfoliative cytologic specimens as well as conventional histologic sections.